This invention presents methods for detection and evaluation of metabolic activity of microorganisms based upon their ability to consume dissolved oxygen. The methods utilize a fluorescence detection system which makes use of the sensitivity of the fluorescent emission of certain compounds to the presence of oxygen, which quenches (diminishes) the compound's fluorescent emission in a concentration dependent manner. Respiring microorganisms will affect the oxygen concentration of a liquid medium in which they are immersed. Thus, this invention provides a convenient system to gather information on the presence, identification and metabolic activity of microorganisms by determining their effect on the oxygen concentration of the media in which they are present.